1. Field of the Invention
This invention concerns an organic electrolyte cell using Li for the anode, manganese dioxide for the cathode and an organic solvent as an electrolyte and, more specifically, it relates to an improvement in the internal resistance of the cell of this kind.
2. Prior Art
It has been known that an organic electrolyte cell using metal lithium with a high electronegativity as an anode active substance and using maganese dioxide as cathode active substance has a cell voltage as high as about 3 V due to the extremely low electrode potential of the metal lithium and it also has a high energy density due to the large electrical capacity of the metal lithium per unit weight. In the cell of this kind, an electrolyte such as lithium perchlorate dissolved in an organic solvent, for example, propylene carbonate is used as the liquid electrolyte and, since both the lithium metal and manganese dioxide are extremely stable in this electrolyte, the cell capacity is not lowered even after a long time storage, which makes the cell extremely desirable in view of the storability.
Accordingly, the application use of the organic electrolyte cell has been increased more and more in recent years as a back-up power source for electronic watches or IC memories that demand high reliability for a long period of time.
However, the aforementioned cell has a drawback that the internal resistance of the cell increases along with the discharge and such internal resistance also increases during storage at high temperature and, accordingly, improvement therefor is a serious problem. The increase in the internal resistance of the cell hinders the effective utilization of the cell. For example, if a large pulse current is taken at the final stage of the discharge, the cell voltage is lowered remarkably due to the high internal resistance and the capacity of the cell can not effectively be utilized to the last in the case of taking out large current pulses.
The cause for the increase in the internal resistance may be attributable to the following act. That is, since the metal lithium is highly active although stable in the organic liquid electrolyte, the surface of the metal lithium gradually reacts with the organic liquid electrolyte to form inactive compound membranes on the surface thereof with the progress of the cell discharge or during storage under high temperature.
In this case, the degree of reaction gives no substantial effects on the cell capacity but, since the membranes formed on the lithium surface greatly hinder the anode reaction of lithium, they increase the internal resistance of the cell.
In view of the above, although the use of lithium-aluminum alloys has been proposed, for example, in U.S. Pat. Nos. 4,002,492 and 4,056,885 for overcoming such drawbacks, no organic electrolyte cells with satisfactory performance have yet been obtained merely by such proposals per se.
Generally, the discharging reaction in the lithium cell using manganese dioxide as the cathode active substance is considered as: EQU Mn.sup.IV O.sub.2 +Li.fwdarw.Li.sup.+ Mn.sup.III O.sub.2
In such a lithium cell, lithium on the anode transfers to the cathode upon discharge and the amount of the discharging products on the cathode is increased along with the discharge to increase the internal resistance. Further, since the opposing area of the lithium electrode is decreased at the final stage of the discharge due to the transfer of the anode lithium, abrupt increase in the internal resistance is caused.